This invention relates to a switching arrangement for ascertaining an overlap or coincidence of digits prior to transfer of code digits being interpreted to the digit interpretation panel of a telephone exchange, more particularly a private branch exchange, wherein a dial control means ascertaining the overlap and a counting chain storing the dialed code digits are provided for each subscriber and/or local or external transfer.
Switching arrangements for private branch exchanges with common control facilities are known in the art, wherein after each dialed code digit there follows a digit interpretation. Only after a complete code has been ascertained, connecting paths are through-connected and other appropriate switching measures taken, such as blocking of the further call setup or, if an exchange code digit is dialed, access is gained to the outside (or exchange) trunk after the first code digit. In this type of system with common control facilities, the digit interpretation, likewise, occurs only simultaneously for a calling and a dialing subscriber station. This principle, commonly known as the "one-at-a-time" principle, can conveniently be employed in systems with common control facilities, since in systems with common control facilities having digit interpreters there are provided selector chains. The selector chains, if there are request signals, ensure that a particular connection to the digit interpreter or to the common control equipment can only take place consecutively. Such selector chains cannot be expanded without special provisions and, in addition, are too costly when their use becomes necessary for each line.
A system has been described in the prior art wherein when one gate is turned on, the other gates of the system are disabled. This gate system is a logic design for a selector apparatus, and it is not designed for preventing the simultaneous transfer of code digits from various selection monitors to a common digit interpretation device. This prior art circuit is described in SEL-NACHRICHTEN, Volume 13, 1965, Number 4, pages 186-197.
An object of the present invention resides in providing simple means for ascertaining the overlap of dialed code digits for each subscriber and/or local or external repeater transfer and in the event there is an overlap for blocking retransmission of code digits for interpretation if overlap occurs without limiting the expansion possibilities of the system.